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Of house wraps and vapor barriers

Q: My house is open to the weather with no trees, only pasture. It was built in 1997 and I bought it in 1998.

About five years ago, I noticed some of the spruce siding was rotting. I replaced it in those areas. The builder used Tyvek under the siding. I remember a while back that your column noted that Tyvek is an incorrect application. I checked your book and did not find the correct underlayment to use, thus my question.

The problems areas are always the west and south sides, and never the east and north sides. Next summer, with retirement in May, my first “keep busy’’ project will be to replace all siding and any rotting underlying particle board in there areas.

So my question is, since Tyvek is not acceptable, what should I use as a vapor barrier?

Also, your advice on three other questions would be helpful:

1. Should I caulk where the siding meets the windows?

2. When I nail the siding, should I avoid nailing the top of the unexposed siding — thus allowing some movement?

3. When I paint the siding, I assume it is best to go lightly where siding meets siding thus allowing for air movement and any moisture to evaporate. Is that correct?

Many thanks for your help. — via email

A: Tyvek is not the problem; Tyvek is a useful house wrap in some cases, but it is not a vapor barrier. Vapor retarders (sometimes erroneously referred to as vapor barriers), which slow moisture penetration, must be installed on the warm side (inside surface) of the exterior walls in most climates. In very hot climates, depending mostly on air conditioning, the vapor retarder is installed on the outside of the exterior walls beneath the siding.

Tyvek is a breathable synthetic air barrier that, when properly used, stops air penetration into the building components over which it is applied. This prevents air movement through even the smallest opening that would reduce the energy efficiency of these building components.

The problems experienced with Tyvek, which I have discussed in the past, are generally caused by the improper installation of the exterior wall cladding.

If Tyvek, or any other synthetic house wrap, is in contact with the cladding, and water from leakage or wind-driven rain penetrates the cladding (a well-known fact), the Tyvek may become saturated and unable to dry. The backside of the cladding remains in contact with the wet house wrap and can begin to deteriorate. The only house wrap I have found not to cause any problem is 30-pound asphalt-saturated felt.

There are other dynamics entering the equation, but we’ll stop there. To break this bond between the house wrap and the cladding, I developed the rain screen in the early 1970s. In those early days, I used 1-inch-by-3-inch furring strips, nailed to every stud over the housewrap to create an air and drainage plane. The bottom and top were screened to keep insects out.

Over the years, the rain screen, although still referred to by that name, has been named differently by other building scientists, but its function has not changed.

The industry has developed several other methods of providing a drainage plane between the house wrap and the cladding. It is also essential to treat the cladding —clapboards in your case — on all sides, including all field cuts. This can be done by back-priming with paint, stain or another form of preservative.

So feel free to use Tyvek or any other synthetic house wrap, but be sure to include the rain screen principle and provide an air space and drainage plane.

The rotting problem on the two windiest and sun-exposed sides of your house also point out what looks like the over-driving of the nails used to fasten the siding. This leaves recessed area around the nails, which absorb water.

There is also rot developing where the clapboards abut window and door frames. There is caulking applied at these joints.

Perhaps the clapboards were installed too tightly and the caulking is simply sitting on top of the clapboards. There are two ways to treat these joints: 1. If the end cuts are thoroughly coated with paint or a preservative, the clapboards can be butted tightly and left uncaulked, or 2. Leave a 1/4-inch space and insert caulking in the joint.

Clapboards should be nailed no more than one inch from the bottom of the boards, so, inevitably the nails will penetrate the lower clapboard. Where two boards meet, they should be butted tightly, but again, be sure that the end cuts are coated.

Henri de Marne’s book, ‘’About the House,’’ is available at www.upperaccess.com and in bookstores. His website is www.henridemarne.com. Readers can send questions to Henri de Marne’s email address at henridemarne@gmavt.net, or to First Aid for the Ailing House, Universal Uclick, 1130 Walnut St., Kansas City, MO 64106.